linux-kernel - Re: [PATCH] x86: coco: mark cc_mask as __maybe

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Message-ID: <20250305221700.GPZ8jNXPCFR1w1NyEQ@fat_crate.local>
Date: Wed, 5 Mar 2025 23:17:00 +0100
From: Borislav Petkov <bp@...en8.de>
To: Ingo Molnar <mingo@...nel.org>
Cc: Arnd Bergmann <arnd@...nel.org>, Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org,
	Kuppuswamy Sathyanarayanan <sathyanarayanan.kuppuswamy@...ux.intel.com>,
	Rick Edgecombe <rick.p.edgecombe@...el.com>,
	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
	Tom Lendacky <thomas.lendacky@....com>,
	Arnd Bergmann <arnd@...db.de>, "H. Peter Anvin" <hpa@...or.com>,
	"Jason A. Donenfeld" <Jason@...c4.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] x86: coco: mark cc_mask as __maybe_unused

On Wed, Mar 05, 2025 at 11:44:34AM +0100, Ingo Molnar wrote:
> 
> * Arnd Bergmann <arnd@...nel.org> wrote:
> 
> > From: Arnd Bergmann <arnd@...db.de>
> > 
> > When extra warnings are enabled, the cc_mask definition in asm/coco.h
> > causes a build failure with gcc:
> > 
> > arch/x86/include/asm/coco.h:28:18: error: 'cc_mask' defined but not used [-Werror=unused-const-variable=]
> >    28 | static const u64 cc_mask = 0;
> > 
> > Mark this one as __maybe_unused.
> > 
> > Fixes: a0a8d15a798b ("x86/tdx: Preserve shared bit on mprotect()")
> > Signed-off-by: Arnd Bergmann <arnd@...db.de>
> > ---
> >  arch/x86/include/asm/coco.h | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> > 
> > diff --git a/arch/x86/include/asm/coco.h b/arch/x86/include/asm/coco.h
> > index aa6c8f8ca958..9e9204cfca6f 100644
> > --- a/arch/x86/include/asm/coco.h
> > +++ b/arch/x86/include/asm/coco.h
> > @@ -25,7 +25,7 @@ u64 cc_mkdec(u64 val);
> >  void cc_random_init(void);
> >  #else
> >  #define cc_vendor (CC_VENDOR_NONE)
> > -static const u64 cc_mask = 0;
> > +static const __maybe_unused u64 cc_mask = 0;
> 
> So I detest __maybe_unused with a vengeance: the 'maybe' unnecessarily 
> inserts uncertainty & probability language into the text, while there's 
> nothing uncertain about this interface or the code. Why cannot the 
> compiler figure it out?
> 
> Anyway, I'd suggest we change direct usage of cc_mask to a 
> get_cc_mask() inline function instead, this will resolve the warning, 
> plus it avoids some messy looking variable shadowing in tdx.c AFAICS:
> 
>   arch/x86/coco/tdx/tdx.c:static void tdx_setup(u64 *cc_mask)

Touch ~17 spots:

$ git grep -w cc_mask arch/x86 | wc -l
17

just because of some stupid gcc extra warning switch?

I say disable the gcc warning. Pfft.

-- 
Regards/Gruss,
    Boris.

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